Lithography is used to pattern semiconductor device structures on wafers. Optimizing the lithography process is increasingly important as feature sizes decrease. For example, in order to accurately replicate a mask or reticle pattern on a substrate, errors in focus should be minimized.
Methods for detecting focus problems in lithography systems include using focus test patterns on a focus test mask. The test patterns are imaged on a wafer and a displacement of one part of the test pattern with respect to another is measured, where the amount and direction of the displacement reflects the amount and direction of defocus (e.g., a positive focus error or negative focus error).
A different method uses a two-exposure process. A test pattern includes one or more line and space patterns and one or more trim patterns, where the wafer is exposed through the line and space pattern during the first exposure, and through the trim pattern during the second exposure.
Like reference symbols in the various drawings indicate like elements.